Beige-tinted glass-ceramics

ABSTRACT

This invention relates to the production of opaque, beige-tinted glass-ceramic articles consisting essentially, in weight percent, of 
     
         ______________________________________                                    
 
    
     SiO 2                                                                  
         66-70         TiO 2                                           
                               3.5-5.5                                    
Al 2  O 3                                                           
         16.5-19.5     CeO 2                                           
                               1.5-2.75                                   
Li 2  O                                                                
         2-4           As 2  O 3                                    
                               0-1.5                                      
MgO      1-5           F       0-1.2                                      
ZnO      0.5-2         ZrO 2                                           
                               0-2.5                                      
Na 2  O                                                                
         0-1                                                              
______________________________________

BACKGROUND OF THE INVENTION

This invention relates to the fabrication of opaque glass-ceramicarticles exhibiting an integral beige tint which are eminently useful asculinary ware.

As is well-recognized in the art, glass-ceramic articles are producedthrough the controlled crystallization of precursor glass articles, theprocess of manufacture normally consisting of three basic steps: first,a glass forming batch typically containing a nucleating agent is melted;second, that melt is cooled to a temperature below the transformationrange thereof and simultaneously shaped into a glass body of a desiredconfiguration; and third, that glass body is exposed to a heat treatmentdesigned to effect the in situ growth of crystals within the glass body.(As is commonly used in the art, the transformation range is defined asthe temperature at which a molten material is transformed into anamorphous mass, that temperature being deemed to reside in the vicinityof the annealing point of a glass.)

Quite frequently, the thermally-induced crystallization in situ will becarried out in two general steps: first, the precursor glass body willbe heated to a temperature slightly above the transformation range for aperiod of time sufficient to generate nuclei therein; and second, thenucleated glass is heated to a temperature approaching, and oftensurpassing, the softening point of the glass to cause the growth ofcrystals on the nuclei. This two-stage heat treatment commonly yieldsglass-ceramic articles containing higher levels of crystallization withmore uniformly-sized, fine-grained crystals. It will be appreciatedthat, as the temperature of the nucleated precursor glass articleapproaches the softening point of the glass, the rate at which thetemperature is raised must be regulated to allow time for the sufficientgrowth of crystallization to resist thermal deformation of the body.Thus, the crystals developed during the heat treatment process are mostusually more refractory than the precursor glass, and thereby canprovide a structure demonstrating resistance to thermal deformation attemperatures higher than those at which the precursor glass can besubjected. Also, because the crystal forming components will have beenremoved therefrom, the small percentage of residual glass remaining inthe glass-ceramic (customarily less than 50% by volume and frequentlyless than 10% by volume) will have a very different composition fromthat of the precursor glass, and most often that residual glass willmanifest a higher softening point than that of the precursor glass.

The development of a high concentration of crystals within aglass-ceramic body has a further advantage vis-a-vis the precursor glassbody in dramatically enhancing the mechanical strength thereof, usuallyby a factor of at least two and frequently as much as three times thatof the precursor glass. That significant improvement in mechanicalstrength, coupled with their substantially higher use temperatures andtheir intrinsic "porcelain-like" appearance, have led to the widespreaduse of glass-ceramic articles as culinary ware.

Generally, in the absence of added colorants, opaque glass-ceramicarticles display a white appearance. For example, Corning Code 9608glass-ceramic, marketed by Corning Incorporated, Corning, N.Y. for over30 years under the trademark CORNING WARE® , exhibits a creamy whiteappearance. Having a composition included within U.S. Pat. No.3,157,522, that opaque glass-ceramic contains a crystallinity in excessof 90% by volume wherein betaspodumene solid solution constitutes thepredominant crystal phase with a minor amount of spinel and rutile alsobeing present. Corning Code 9608 has the following approximate analysis,expressed in terms of weight percent on the oxide basis:

    ______________________________________                                        SiO.sub.2                                                                             69.5      ZnO     1.0    F     0.03                                   Al.sub.2 O.sub.3                                                                      17.7      TiO.sub.2                                                                             4.7    Fe.sub.2 O.sub.3                                                                    0.05                                   Li.sub.2 O                                                                             2.7      ZrO.sub.2                                                                             0.2    B.sub.2 O.sub.3                                                                     0.07                                   MgO      2.6      As.sub.2 O.sub.3                                                                      0.6    MnO.sub.2                                                                           0.03                                   ______________________________________                                    

As might well be expected, colorants known in the glass art have beenincorporated into precursor glass compositions which have subsequentlybeen crystallized in situ to glass-ceramic articles. U.S. Pat. No.4,461,839 (Rittler) and U.S. Pat. No. 4,786,617 (Andrieu et al.) arerecent illustrations of that practice.

The former patent discloses the manufacture of opaque glass-ceramicarticles containing β-spodumene solid solution as the predominantcrystal phase, which can display colors ranging from gray to brown toalmond to beige to yellow to blue, that are prepared from precursorglass articles having base compositions essentially free from MgO andconsist essentially, in weight percent, of:

    ______________________________________                                        SiO.sub.2                                                                              63.5-69       BaO     0-5                                            Al.sub.2 O.sub.3                                                                       15-25         TiO.sub.2                                                                             2-3                                            Li.sub.2 O                                                                             2.5-4         ZrO.sub.2                                                                             0.5-2.5                                        Na.sub.2 O                                                                             0.1-0.6       As.sub.2 O.sub.3                                                                      0.4-0.8                                        K.sub.2 O                                                                              0.1-0.6       Fe.sub.2 O.sub.3                                                                      0.05-0.1                                       ZnO      0-2                                                                  ______________________________________                                    

The desired colors are obtained through the use of a "color package"containing about 0.5-3% TiO₂ and up to 0.15% Fe₂ O₃ with 0.3-3% total ofat least two oxides in the indicated proportion selected from the groupof up to 0.3% V₂ O₅, up to 3% CeO₂, up to 2% CaO, up to 1% NiO, up to 1%WO₃, and up to 1.5% SnO₂. The total TiO₂ content in the glass willrange > 2.5-6% and that of the Fe₂ O₃ content will range 0.05-0.2%.

The latter patent describes the fabrication of opaque glass-ceramicarticles containing potassium fluorrichterite and/or a related fluormicaas the predominant crystal phase(s) from precursor glass compositionsessentially free from Li₂ O and which consist essentially, in weightpercent, of:

    ______________________________________                                        SiO.sub.2                                                                             61-70        K.sub.2 O  2.5-5.5                                       Al.sub.2 O.sub.3                                                                      2.75-7       Na.sub.2 O + K.sub.2 O                                                                   <6.8                                          MgO     11-16        F            2-3.25                                      CaO     4.75-9       BaO          0-3.5                                       Na.sub.2 O                                                                            0.5-3        P.sub.2 O.sub.5                                                                            0-2.5                                       ______________________________________                                    

The text of the patent noted that it was possible to incorporate suchconventional glass colorants as Fe₂ O₃, CeO₂, CaO, Cr₂ O₃, CuO, MnO₂,Na₂ O, and V₂ O₅ into the base precursor glass composition in amountstypically less than 1% total. Nevertheless, only the use of Fe₂ O₃ toimpart a yellow tint to the glass-ceramic was expressly mentioned.

Corning Incorporated currently markets a line of opal glass tablewareunder the trademark CORNERSTONE® . That product has a compositionincluded in U.S. Pat. No. 4,331,769 (Danielson et al.) and exhibits abeige tint defined within the polygon bounded by Points ABCDEFA depictedin the appended drawing, which polygon encompasses a plot of the x and ychromaticity coordinates (Illuminant C). The visual appearance of theproduct is described here in accordance with the standard CIE systemutilizing chromaticity coordinates x and y and the tristimulus value Y.Thus, the values are measured under standard conditions, i.e.,Illuminant C, with a Hunter Colorimeter and represent the light thatdiffusely reflects off opaque surfaces. Because the values obtained arereadily reproducible, they are commonly employed to facilitatecomparisons and to delimit specifications.

The tint is imparted to the CORNERSTONE® tableware through theincorporation of NiO into the base glass compositions. The researchleading to the present invention had as its goal the development of anopaque glass-ceramic body demonstrating properties suitable for use ascookware which would exhibit a hue close to and compatible with that ofCORNERSTONE® tableware, thereby offering to the consumer market acomplete line of dinnerware and cookware of approximately the same tint.Because the glass-ceramics were destined for use as culinary ware, thechemical and physical properties recognized in the art as beingnecessary in such articles would likewise be required in the tintedarticles. For example, the tinted articles would exhibit low linearcoefficients of thermal expansion, viz., < 15 and preferably < 13×10⁻⁷/°C. over the temperature range of 0°-300°C., and good resistance to thechemical attack of food products.

SUMMARY OF THE INVENTION

In view of the fact that Corning Code 9608 was known to manifest thechemical and physical properties to perform well as cookware, additionsof known coloring agents, both individually and in various combinations,were made to the base precursor or glass composition therefor inattempts to substantially duplicate the hue of CORNERSTONE® tableware.As a result of those testing experiments, we discovered a very narrowrange of precursor base glass compositions containing about 1.5-2.75%CeO₂ as the colorant which could be heat treated to form opaqueglass-ceramic articles demonstrating the chemical and physicalproperties required in glass-ceramics devised for use as culinary ware,and which also display a beige tint close to and compatible with that ofCORNERSTONE® tableware. (The incorporation of NiO in the precursor baseglass composition of Corning Code 9608 imparts a blue coloration to theglass-ceramic.) Those glass compositions consist essentially, expressedin terms of weight percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                              66-70         TiO.sub.2                                                                             3.5-5.5                                        Al.sub.2 O.sub.3                                                                       16.5-19.5     CeO.sub.2                                                                              1.5-2.75                                      Li.sub.2 O                                                                             2-4           As.sub.2 O.sub.3                                                                        0-1.5                                        MgO      1-5           F         0-1.2                                        ZnO      0.5-2         ZrO.sub.2                                                                               0-2.5                                        Na.sub.2 O                                                                             0-1                                                                  ______________________________________                                    

In like manner to conventional glass-ceramics known to the art, theproducts of the present invention are prepared in accordance with thefollowing three general steps:

(a) a batch for a glass having a composition within the above ranges ismelted;

(b) that melt is cooled to a temperature below the transformation rangethereof and simultaneously a glass body of a desired configuration isshaped therefrom; and

(c) the glass body is heat treated in a manner to effect thecrystallization in situ thereof.

To assure the development of a highly crystalline article wherein thecrystals are uniformly fine-grained, the precursor glass body will benucleated via exposure for a sufficient period of time to a temperaturewithin the range of about 750°-850°C., and thereafter will becrystallized via exposure for a sufficient period of time to atemperature within the range of about 1025°-1175°C. Whereas an expressdwell period within either of those temperature ranges may be used as amatter of convenience, such is not required. It is only necessary thatthe glass article be within those ranges for a sufficient length of timeto accomplish the purpose therefor. As can be appreciated, the timerequired is dependent in some measure upon the thickness of the glassarticles being heat treated to assure temperature equilibrationthroughout the body thereof.

The inventive glass-ceramics are very highly crystalline, i.e., greaterthan 75% by volume crystalline and, more desirably, greater than 90% byvolume crystalline. The crystals, themselves, are quite uniformlyfine-grained, with diameters of less than 1 micron. β-spodumene solidsolution (s.s.) constitutes by far the predominant crystal phase withminor levels of spinel and rutile. The cerium-containing mineralsloparite (Ce₂ Ti₃ O₈.7) and perrierite (Ce₂ Ti₂ Si₂ O₁₁) have also beenidentified via x-ray diffractometry as being present in small amounts.

In summary, to achieve the beige tint displayed by CORNERSTONE®dinnerware, the above-recited composition intervals for the inventiveglass-ceramics must be very closely observed. Moreover, care must alsobe observed in the crystallization heat treatment applied to theinventive precursor glass articles, as will be illustrated hereinafter.

BRIEF DESCRIPTION OF THE DRAWING

The drawing depicts two polygons encompassing plots of reflectancechromaticity coordinates x and y (Illuminant C) measured on CORNERSTONE®tableware and articles of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Table I below lists several precursor glass compositions, expressed interms of parts by weight on the oxide basis as calculated from thebatch, capable of being crystallized in situ to yield opaqueglass-ceramic articles, which compositions illustrate the compositionparameters of the present invention. Inasmuch as the sum of theindividual components closely approximates 100, for all practicalpurposes the values recorded may be deemed to represent weight percent.Because it is not known with which cation(s) the fluoride is combined,and the amount present to act as a melting aid and as a strengtheningagent as disclosed in U.S. Pat. No. 3,148,994 is small, it is merelyreported as fluoride in accordance with conventional glass analysispractice. Fe₂ O₃ was not intentionally included in the composition butis present therein as an impurity from the batch materials, particularlyin the sand comprising the source of SiO₂. Because of its severe adverseeffect upon color, Fe₂ O₃ will preferably be essentially absent from thecompositions and should be held below 0.05% by weight. The actualingredients employed in preparing the glass forming batch may compriseany materials, either the oxides or other compounds, which, when meltedtogether, will be converted into the desired oxide in the properproportions. For example, petalite may be used to supply Li₂ O, Al₂ O₃,and SiO₂. Sodium silico-fluoride provided the source of fluoride in thetwo compositions tabulated below. Arsenic oxide was included to performits conventional function as a fining agent.

                  TABLE I                                                         ______________________________________                                              1     2          3     4      5     6                                   ______________________________________                                        SiO.sub.2                                                                           69.60 69.35      69.10 68.85  68.60 68.35                               Al.sub.2 O.sub.3                                                                    17.83 17.83      17.83 17.83  17.83 17.83                               TiO.sub.2                                                                           4.70  4.70       4.70  4.70   4.70  4.70                                Li.sub.2 O                                                                          2.70  2.70       2.70  2.70   2.70  2.70                                MgO   2.60  2.60       2.60  2.60   2.60  2.60                                ZnO   1.04  1.04       1.04  1.04   1.04  1.04                                As.sub.2 O.sub.3                                                                    0.52  0.52       0.52  0.52   0.52  0.52                                Na.sub.2 O                                                                          0.37  0.37       0.37  0.37   0.37  0.37                                F     0.027 0.027      0.027 0.027  0.027 0.027                               Fe.sub.2 O.sub.3                                                                    0.033 0.033      0.033 0.033  0.033 0.033                               CeO.sub.2                                                                           0.50  0.75       1.00  1.25   1.50  1.75                                ______________________________________                                              7         8       9        10    11                                     ______________________________________                                        SiO.sub.2                                                                           68.10     67.60   67.10    68.20 68.38                                  Al.sub.2 O.sub.3                                                                    17.83     17.83   17.83    18.80 17.84                                  TiO.sub.2                                                                           4.70      4.70    4.70     3.70  4.70                                   Li.sub.2 O                                                                          2.70      2.70    2.70     2.90  2.70                                   MgO   2.60      2.60    2.60     2.20  2.60                                   ZnO   1.04      1.04    1.04     1.04  1.04                                   As.sub.2 O.sub.3                                                                    0.52      0.52    0.52     0.63  0.63                                   Na.sub.2 O                                                                          0.37      0.37    0.37     0.37  0.37                                   F     0.027     0.027   0.027    0.03  0.027                                  ZrO.sub.2                                                                           --        --      --       0.10  --                                     Fe.sub.2 O.sub.3                                                                    0.033     0.033   0.033    0.048 0.048                                  CeO.sub.2                                                                           2.00      2.50    3.00     1.80  1.86                                   ______________________________________                                    

The batch ingredients were compounded, ballmilled together to assist insecuring a homogeneous melt, and then discharged into platinumcrucibles. The crucibles were moved into a furnace operating at about1625°C. and the batches melted overnight (˜ 16 hours). the melts werestirred slowly, poured and squeezed through stainless steel rollers toproduce glass patties having a width of about 10-15 cm, a length ofabout 25 cm, and a thickness of 1.5 cm, and those patties were annealedat 700°C. The annealed patties were cut into shapes suitable for testingpurposes.

It will be appreciated that the above glass melting and formingprocesses reflect laboratory practice only. Stated in another way, theabove glasses are capable of being melted and formed utilizingcommercial, large scale glass melting and forming equipment, and are notlimited to laboratory activity. Furthermore, although the compositionsof Table I were annealed to room temperature to permit examination ofglass quality and to cut test samples from the patties, that action isnot required. It is only necessary that the batches be heatedsufficiently to produce a homogeneous melt, that melt cooled to atemperature below the transformation range thereof to yield anessentially crystal-free glass, and that glass body then subjected tothe nucleation-crystallization heat treatment to convert it into aglass-ceramic.

Table II reports approximate heat treatment schedules which wereemployed with the glass samples of Table I along with the linearcoefficient of thermal expansion (Exp) as measured over the temperaturerange of 0°-300°C. in terms of ×10⁻⁷ /°C., and the x and y colorcoordinates with tristimulus value Y utilizing Illuminant C.Temperatures are listed in °C. and time in hours (hr).

                  TABLE II                                                        ______________________________________                                        Ex.     Heat Treatment Exp    x     y     Y                                   ______________________________________                                        1       25-700 at 1000/hr                                                                            --     0.3212                                                                              0.3315                                                                              88.22                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     2       25-700 at 1000/hr                                                                            --     0.3253                                                                              0.3333                                                                              85.04                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     3       25-700 at 1000/hr                                                                            --     0.3253                                                                              0.3342                                                                              83.64                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     4       25-700 at 1000/hr                                                                            11.5   0.3268                                                                              0.3353                                                                              79.66                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     5       25-700 at 1000/hr                                                                            11.9   0.3286                                                                              0.3376                                                                              79.0                                        700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     6       25-700 at 1000/hr                                                                            12.1   0.3306                                                                              0.3398                                                                              78.46                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     7       25-700 at 1000/hr                                                                            12.3   0.3335                                                                              0.3428                                                                              76.77                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     8       25-700 at 1000/hr                                                                            11.1   0.3336                                                                              0.3430                                                                              74.58                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     9       25-700 at 1000/hr                                                                            12.0   0.3368                                                                              0.3462                                                                              73.13                                       700-820 at 265/hr                                                             820-850 at 30/hr                                                              850-1100 at 240/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     10      25-800 at 800/hr                                                                             11.5   0.3323                                                                              0.3376                                                                              63.5                                        800-830 at 36/hr                                                              830-1070 at 240/hr                                                            1070-1080 at 15/hr                                                            1080-25 at 1050/hr                                                    11      25-700 at 1000/hr                                                                            13.1   0.3306                                                                              0.3377                                                                              77.97                                       700-800 at 215/hr                                                             800-835 at 30/hr                                                              835-1100 at 230/hr                                                            1100 for 1 hr                                                                 1100-950 at 400/hr                                                            950-25 at 2500/hr                                                     ______________________________________                                    

As can be seen from Table II, although no express dwell period withinthe nucleation range was employed, the rate of temperature increasethrough the 750°-850°C. range was controlled such that the samples werewithin that temperature interval for about 1.25 hours. Longer timeswithin the nucleation range can be used without concern with the samplesto achieve even greater nucleation, but are unnecessary and areuneconomical from a practical point of view, inasmuch as sufficientnucleation was generated within that period to assure the extensivegrowth of uniformly-sized, very fine-grained crystals. Hence, exposureperiods of about 1-3 hours have been deemed satisfactory.

At temperatures below about 1025°C., growth of crystals is quite slow.On the other hand, crystallization temperatures in excess of about1175°C. can lead to grain growth of the crystals and thermal deformationof the body. It will be appreciated that, with articles of similarphysical dimensions, longer exposure periods to assure the developmentof a high level of crystallinity will be required at lower temperatureswithin the crystallization range than at temperatures at the upperextreme thereof. Whereas in the heat treatment schedules listed in TableII the samples remained within the crystallization range for about1-1.25 hours, longer exposure periods can be utilized without concern,but such longer periods can be uneconomical from a practical point ofview in like manner to very extended nucleation periods. Accordingly,crystallization heat treatments of about 1-4 hours have been consideredsufficient to achieve the desired extensive growth of uniformly-sized,very fine-grained crystals.

The x and y coordinates for Examples 1-11 are positioned within polygonbounded by Points A'B'C'D'E'F'A', which polygon overlaps the plot of thex and y coordinates encompassed with polygon bounded by Points ABCDEFAand, hence, includes tints not only matching that of CORNERSTONE®tableware, but also hues close to and compatible therewith. As isevident from polygon A'B'C'D'E'F'A', the tints of Examples 1-4,containing CeO₂ concentrations of 0.5%, 0.75%, 1%, and 1.25%,respectively, fall outside of the desired values, and Example 5,containing 1.5% CeO₂, is at the edge of acceptable color. Accordingly,1.5% CeO₂ has been deemed a practical minimum limit. At the otherextreme, Example 9, containing 2.5% CeO₂ is close to the edge ofacceptable color and Example 9, containing 3% CeO₂ is outside ofacceptable color. Therefore, 2.75% CeO₂ has been adjudged an appropriatemaximum level, with 1.75-2.25% CeO₂ being the preferred range.

To further illustrate the singular behavior of CeO₂ in imparting a beigetint close to and compatible with the hue of CORNERSTONE® tableware,CeO₂ was substituted for SiO₂ in amounts of 1.5%, 2.0%, and 2.5% in thebase composition of the CORNERSTONE® tableware. The batches werecompounded, ballmilled, melted at 1550°C., poured into 6"×6"×1/2" steelmolds, and annealed at 600°C. in like manner to the procedure describedin Patent No. 4,331,769. The compositions of those three glasses arerecorded below in Table III, expressed in terms of parts by weight onthe oxide basis as calculated from the batch. Because the sum of theindividual constituents closely approximates 100, for all practicalpurposes the tabulated values may be deemed to reflect weight percent.

                  TABLE III                                                       ______________________________________                                                12          13     14                                                 ______________________________________                                        SiO.sub.2 62.9          62.4   61.7                                           Al.sub.2 O.sub.3                                                                        6.28          6.28   6.31                                           Na.sub.2 O                                                                              3.04          3.04   3.05                                           B.sub.2 O.sub.3                                                                         4.86          4.86   4.89                                           CaO       15.2          15.2   15.3                                           MgO       1.01          1.01   1.01                                           KaO       3.04          3.04   3.05                                           F         3.55          3.55   3.56                                           CeO.sub.2 1.50          2.0    2.51                                           ______________________________________                                    

The x and y color coordinates tristimulus values Y (Illuminant C) forthose three glasses were measured as follows:

    ______________________________________                                        Glass 12     x = 0.3201, y = 0.3282, Y = 77.0                                 Glass 13     x = 0.3213, y = 0.3281, Y = 72.5                                 Glass 14     x = 0.3282, y = 0.3342, Y = 67.9                                 ______________________________________                                    

As is immediately evident, those values fall far outside of polygonA'B'C'D'E'F'A' and, hence, the hues would not be compatible with thetint of CORNERSTONE® tableware.

The more preferred composition intervals yielding tints not only closelymatching that of CORNERSTONE® tableware, but also exhibiting physicaland chemical properties rendering them exceptionally suitable forculinary ware, consist essentially, expressed in terms of weight percenton the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                              68.0 ± 2.0 TiO.sub.2                                                                              4.5 ± 0.75                                 Al.sub.2 O.sub.3                                                                       18.0 ± 1.5 CeO.sub.2                                                                              2.0 ± 0.25                                 Li.sub.2 O                                                                             2.75 ± 0.5 As.sub.2 O.sub.3                                                                      0.75 ± 0.5                                  MgO       2.5 ± 0.75                                                                              F        0.5 ± 0.48                                 ZnO       1.0 ± 0.25                                                                              ZrO.sub.2                                                                               0 - 0.5                                      Na.sub.2 O                                                                              0.5 ± 0.25                                                       ______________________________________                                    

Example 10 is considered to be the most preferred composition.

We claim:
 1. A beige-tinted, opaque glass-ceramic article containingβ-spodumene solid solution as the predominant crystal phase with minoramounts of rutile, spinel, loparite, and perrierite which exhibits alinear coefficient of thermal expansion (0°-300°C.) less than 15×10⁻⁷/°C. and x and y color coordinates (Illuminant C) coming within polygonbounded by Points A'B'C'D'E'F'A' of the drawing, said glass-ceramichaving a composition consisting essentially, expressed in terms ofweight percent on the oxide basis, of

    ______________________________________                                        SiO.sub.2                                                                              66-70         TiO.sub.2                                                                             3.5-5.5                                        Al.sub.2 O.sub.3                                                                       16.5-19.5     CeO.sub.2                                                                             1.5-2.75                                       Li.sub.2 O                                                                             2-4           As.sub.2 O.sub.3                                                                      0-1.5                                          MgO      1-5           F       0-1.2                                          ZnO      0.5-2         ZrO.sub.2                                                                             0-2.5                                          Na.sub.2 O                                                                             0-1                                                                  ______________________________________                                    


2. A glass-ceramic article according to claim 1 consisting essentially,in weight percent, of

    ______________________________________                                        SiO.sub.2                                                                              68.0 ± 2.0 TiO.sub.2                                                                              4.5 ± 0.75                                 Al.sub.2 O.sub.3                                                                       18.0 ± 1.5 CeO.sub.2                                                                              2.0 ± 0.25                                 Li.sub.2 O                                                                             2.75 ± 0.5 As.sub.2 O.sub.3                                                                      0.75± 0.5                                   MgO      2.5 ± 0.5  F        0.5 ± 0.48                                 ZnO       1.0 ± 0.25                                                                              ZrO.sub.2                                                                               0 - 0.5                                      Na.sub.2 O                                                                              0.5 ± 0.25                                                       ______________________________________                                    